Locating Planet Mercury

Dia. 1: The Spring Evening Sky. Diagram shows the position of the ecliptic in relation to the horizon as seen from Orkney April 21 2008 at 20h UT. Since Mercury will appear to move against the stellar backgound along the line of the ecliptic in the direction away from the horizon it will be seen that maximum advantage is gained from the high angle inclination of the former with the latter. The position is reversed for the morning sky.

Dia. 2 The Autumn Morning Sky. Diagram shows the position of the ecliptic in relation to the horizon as seen from Orkney October 22nd 2008 at 06h UT. Since Mercury will appear to move against the stellar backgound along the line of the ecliptic in the direction away from the horizon it will be seen that maximum advantage is gained from the high angle inclination of the former with the latter. The position is reversed for the evening sky.

Dia. 3. The late evening sky looking NW on May 6th 2008 at 22h 00m UT (11pm BST) showing the crescent moon above Mercury.

Orbiting close to the Sun, Mercury is generally only observable to the unaided eye relatively close to greatest elongations. Venus, in contrast, may be seen at times without optical assistance even at conjunctions with the Sun. This is because Venus may appear at conjunction with elongations (above or below the Sun) of up to 8°. When this occurs above the Sun from a given location it is possible to see Venus as both a morning and evening “star” on one and the same day. For example, on January 16 1998, with Venus a little under 7° above the Sun, it was possible to see the planet in the morning and evening twilight as witnessed here on Rousay.

Venus (in view of the nature of its atmosphere producing a high albedo) even at inferior conjunction, with a phase of as little as 1%, may appear as bright as –4.0 magnitude. This will in fact occur on March 27 next year (2009).

Mercury, on the other hand, is at its faintest close to inferior conjunctions when it is difficult if not impossible even in large telescopes.

Mercury is always at it brightest close to or at superior conjunction (maximum magnitude –2.3). Then, the surface brightness per unit area may exceed that of Venus. However, Mercury at superior conjunction is never more than a little over 2° from the Sun. Mercury is not therefore visible to the unaided eye as both a morning and evening object on one and the same date.

In general the best time to look for Mercury with the unaided eye is therefore close to elongations. From the northern hemisphere evening observations are best made in spring at eastern elongations and in the autumn, before dawn, at western elongations (see Dias. 1 &2).

As already noted, Mercury’s brightness varies over a far greater range than does Venus’s. Since Mercury is at its brightest at superior conjunction, the brightness falls off as it proceeds towards greatest eastern elongation and beyond.

Thus this year (2008), a superior conjunction occurs on April 16. Mercury reaches greatest eastern elongation (22°) on May 14, when its magnitude will be 0.4, similar to that of the bright star Procyon. (This will also see the planet close to greatest declination north at +25.1°.) A week earlier, on the 7th May, Mercury will be twice as bright at magnitude –0.4.

In practical terms it should be possible to see Mercury from around April 25 when it will be at –1.5 magnitude, a little brighter than Sirius. Thereafter Mercury moves rapidly eastward against the star background - fading the wile - to be a little below the Pleiades cluster on May 1. The thin crescent Moon will pass above the planet on May 6.

On 20 May, at magnitude 1.3, Mercury will set a few minutes after 23 hrs UT (midnight BST). It is unlikely to be visible to the unaided eye much after this date.

JV 10/04/08